Heat shrinkable polymer films have gained substantial acceptance for such uses as the packaging of meats. This description will detail the usage of films for packaging meats; it being understood that these films are also suitable for packaging other food products. Some of the films embodying this invention are typically used as heat shrinkable bags supplied to the meat packer with one open end, to be closed and sealed after insertion of the meat. After the produce is inserted, air is normally evacuated, the open end of the bag is closed, such as by heat sealing, or applying a metal clip, and finally heat is applied, such as by hot water, to initiate film shrinkage about the meat.
In subsequent processing of the meat, the bag may be opened and the meat removed for further cutting of the meat into user cuts, for retail sale, for example, or for institutional use.
Successful shrink bags must satisfy a multiplicity of requirements imposed by both the bag producer and the bag user. The bag must be strong to protect the meat. However, it is also desirable to have a package that is attractive. After shrinking, the tighter the fit of the package film to the meat, the better the appearance. A tight fit eliminates gapping areas where unsightly purge can collect. On the other hand, the tight fit must not cause the bag to exhibit such a level of shrink energy as would cause the film to pull apart or delaminate during shrinkage, under its own internal forces.
Conventional shrink bags have generally been constructed with ethylene vinyl acetate copolymers (EVA). In some cases the bags contain a layer of a copolymer to serve as an oxygen barrier. Ethylene vinyl alcohol copolymer (EVOH) has been suggested as the barrier layer, and so has polyvinylidene chloride copolymer (PVDC).
A heat shrinkable, thermoplastic, barrier packaging film for making bags which has enjoyed considerable commercial success is described in U. S. Pat. No. 3,741,253 issued on June 26, 1973 to Brax et al, which relates to a multilayer film comprising a first outside layer of an irradiated ethylene-vinyl acetate copolymer, a core layer of vinylidene chloride copolymer, and a second outside layer of an ethylene-vinyl acetate copolymer. In manufacturing this type of heat shrinkable film, a tubular orientation process is utilized wherein a tube of the film is biaxially oriented by stretching with internal pressure in the transverse direction and with the use of pinch rolls at different speeds in the machine direction. Then the bubble is collapsed, and the film is wound up as flattened, seamless, tubular film to be used later to make bags, e.g. either end-seal bags typically made by transversely heat sealing across the width of flattened tubing followed by severing the tubing so that the transverse seal forms the bottom of a bag, or side-seal bags in which the transverse seals form the bag sides and one edge of the tubing forms the bag bottom.
This type of bag is typically used by placing the food product in the bag, evacuating the bag, gathering and applying a metal clip around the gathered mouth of the bag to form a hermetic seal, or heat sealing the mouth end of the bag, and then immersing the bag, typically for a few seconds in a hot water bath at approximately the same temperature at which the film was stretch-oriented, typically about 160.degree. to 205.degree. F. (71.1.degree. C. to 96.1.degree. C.), hot water immersion being one of the quickest and most economical means of transferring sufficient heat to the film to shrink it uniformly.
One use of such bags is in the shrink packaging of hams. Moist meat, particularly moist ham, can be an excellent growing environment for Listeria, a bacterium that can cause pregnant women to miscarry or cause encephalitis. Thus, ham is commonly subjected to drying to reduce the moisture content thereof prior to shrink packaging. However, when the drier hams are shrink packaged, the bag as it shrinks cannot slip across the drier ham as easily as it could across a moister ham. The product to container fit is not as tight as desired. Gapping areas occur where unsightly purge can collect. Attempts have been made to reduce the gap. One such attempt has been by holding a packaged ham for 0.5 hour or so prior to hot water shrinking of the bag to allow residual moisture inside the ham to permeate to the ham surface. This somewhat reduces the gap but sometimes the same amount of gap occurs whether the package has been held or not. Moreover, the holding time increases the packaging time, and thus fewer hams per unit time can be packaged. This increases the cost of shrink packaged ham. Furthermore, the gap is never completely eliminated.
Thus, it is desired to find a way which reduces the gap so that it is completely eliminated or which once in a while leaves a very minimal gap, yet which is not very time consuming.
Accordingly, it is a general object of the present invention to provide a shrink package wherein undesirable gaps between the shrunken package film and the food product wrapped therein are substantially prevented.